A return flow prevention arrangement in a draw-off tap with an extensible outlet nozzle for a sink basin is known from U.S. Pat. No. 4,696,322. This draw-off tap includes a mixing valve with a hollow sphere-shaped valve body in the tap housing. The housing has two supply connections and one mixed water outlet channel. A diaphragm-like ventilating valve, which opens into the mixed water channel within the mixing valve body, is built into the valve body of the mixing valve. The air supply openings to the ventilating valve are arranged on the upper side of the mixing valve body. If one of the supply lines has reduced pressure, air is sucked in through the ventilating valve and thus water is prevented from being sucked in from the outlet nozzle. This is advantageous because the extensible nozzle can lie, under some circumstances, in the sink basin filled with dirty water.
The drawbacks with this known return flow safety valve arrangement are that the return flow of the water into the supply lines cannot be reliably prevented by the ventilating valve alone, and the ventilating valve is not accessible for maintenance. When it is defective, the entire mixing valve body must be replaced, which is quite expensive. In addition, it is almost impossible to check the functionality of the ventilating valve.